Single Radio Voice Call Continuity (SRVCC) is a functionality that allows a VoIP (Voice over IP) or an IMS (IP Multimedia Subsystem) call in the packet domain, such as an LTE domain to be moved to a legacy circuit switched voice domain, such as GSM/UMTS or CDMA 1×.
As an example, SRVCC may function as follows: A user equipment (UE) that is SRVCC capable and that accesses an LTE network is engaged in a voice call. When the UE leaves the LTE coverage area it may be determined that the voice call is to be handed over to a legacy circuit switched domain and the voice call is then transferred from LTE to the circuit switched network.
According to 3GPP TS 23.216 V11.4.0 Single Radio Voice Call Continuity (SRVCC) may relate to voice call continuity between IMS (IP Multimedia Subsystem) over PS (Packet Switched) access and CS (Circuit Switched) access for calls that are anchored in IMS when the UE (User Equipment) is capable of transmitting/receiving on only one of those access networks at a given time.
Single Radio Video Call Continuity (vSRVCC) relates to video call continuity from E-UTRAN (evolved UMTS Terrestrial Radio Access Network) to UTRAN-CS for calls that are anchored in the IMS when the UE is capable of transmitting/receiving on only one of those access networks at a given time. The term vSRVCC refers to Single Radio Video Call Continuity.
Single Radio Voice Call Continuity (SRVCC) is discussed and specified in 3GPP TS 23.216 V11.4.0, entitled Single Radio Voice Call Continuity (SRVCC) and in 3GPP TS 23.237 V11.4.0, entitled IP Multimedia Subsystem (IMS) Service Continuity. IMS SIP (Session Initiation Protocol) signaling is discussed in 3GPP TS 23.228 V11.4.0, entitled IP Multimedia Subsystem. User Equipment Radio Capability Match procedures are discussed in 3GPP TS 23.401 V11.1.0 entitled General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access and in 3GPP TS 23.060 V11.1.0 entitled General Packet Radio Service (GPRS), Service description.
While SRVCC often refers to the handover of a session from the packet switched domain to the circuit switched domain, in this application the opposite direction of a session transfer is considered, i.e. session transfer from the circuit switched domain to the packet switched domain will be the main focus. The concept of SRVCC from circuit switched to packet switched will thus also be referred to as rSRVCC, meaning return SRVCC or reverse SRVCC.
The current 3GPP specification TS23.216 V11.4.0 discusses in Section 5.3.2b an MSC (Mobile Switching Centre) enhanced for UTRAN/GERAN to E-UTRAN/UTRAN (HSPA) SRVCC; (UTRAN: UMTS Terrestrial Radio Access Network, GERAN: GSM EDGE Radio Access Network, E-UTRAN: evolved UTRAN, HSPA: High Speed Packed Access). According to this section, the MSC may inform the RAN (Radio Access Network) about the possibility to perform CS (Circuit Switched) to PS (Packet Switched) SRVCC by sending a “CS to PS SRVCC operation possible” to the RNC (Radio Network Controller) or alternatively to the BSC (Base Station Controller) only for normal TS 11 call (i.e. not an emergency call—TS 12, 3GPP TS 22.003). The MSC Server determines the possibility of performing CS to PS SRVCC based on: the CS to PS SRVCC capability of the UE, the presence of the CS to PS SRVCC allowed indication in the subscription data of the user, and the IMS registration status of the UE.
According to existing specification, the MSC Server indicates to the RAN that rSRVCC (SRVCC from CS to PS) is supported even if the UE does not support the actually required capabilities. For example, the network supports rSRVCC from UTRAN to LTE but the UE may only supports rSRVCC from GERAN to LTE. In such cases a handover or session transfer from the CS domain to the PS may fail or the call may even be lost or need to be recovered.